Thanks everyone. Mangled replies...
I've got the service manuals and I've fixed it too many times now
. The transformer is on the way out (insulation decay), the tube has faded so you can see it in the dark on high brightness (even though voltages are ok) and the timebase capacitors are on the way out as are the pots. It's death by a thousand paper cuts now so I'm going to have to retire it. I paid £20 for it about 20 years ago and it was 20 years old then!
Trying to get hold of something I can prop up for another 20 years hence no proprietary ICs. The D83 didn't have ANY ICs in it. I don't mind standard op amps, regulators, logic but nothing more. I had a friend with a 2225 that blew something expensive a few years ago. Quite reliable until they're not and the thing with the burn marks on it was a 14 pin DIP with "Tek" written on it.
465 looks to be a good bet so far.
After analogue because I tend to work entirely in the sub-50MHz analogue domain (as it's interesting).